I worry that being in the same class as a child with a disability affects lessons or the teacher's attention to other children.

It's natural to worry. In many schools, support staff and accommodations are arranged so that other children's learning isn't harmed. For points that concern you, rather than judging by guesswork, asking the teacher concretely at a conference or in the home-school notebook brings reassurance. Reasonable accommodation often helps not only a specific child but the whole class's ease of learning.